Over 6 million tons of phenolic resins are produced each year globally. They are relatively inexpensive and possess excellent properties, making them suitable for a wide range of applications. Phenolic resins exhibit good heat resistance, high mechanical strength, electrical insulation, excellent creep resistance, good processability, and flame resistance.
Phenol-formaldehyde resins generally occur in two forms: resole (viscous liquid) and novolac (solid). Resoles have a formaldehyde to phenol ratio of greater than one. Resoles are self-curing without requiring any cross-linkers, and can be easily compounded with additives, fillers, or fibers. Curing of resole occurs by the loss of 1.5 moles of formaldehyde per mole of resole, which is usually released as a toxic gas. The shelf-life of resole is 6-8 months when stabilized and stored under ideal conditions. Novolacs, on the other hand, have a formaldehyde-to-phenol molar ratio of less than one. Novolacs require curing agents and is usually mixed with hexamine derivatives.
Several curing agents for novolac resins are known in the art, including formaldehyde, paraformaldehyde and hexamethylenetetramine. The most common curing agent is hexamethylenetetramine, which reacts upon heating to yield ammonia, formaldehyde, methylene amine derivatives, and a cured resin. These curing agents complete the cross-linking reaction to convert a thermoplastic novolac resin to an insoluble infusible state. However, each of these novolac curing agents has certain disadvantages. For instance, where hexamethylenetetramine or formaldehyde are used to cure a novolac resin, volatile reaction products are emitted during the cure reaction. Specifically, when the curing agent is hexamethylenetetramine, toxic gas such as ammonia and formaldehyde are released during curing of the novolac resin. Further, hexamethylene derivatives are highly explosive, and require storing the cured resins under temperature regulated conditions, which are not economical. In addition, novolac curing agents like hexamethylenetetramine typically require curing temperatures as high as 150° C. Cure temperatures can be lowered by the addition of acids, but this often introduces other problems such as die staining, die sticking and sublimation of organic acids into the atmosphere.
Whilst both of these phenolic resins are used ubiquitously, the issues outlined above concerning their handling, storage and the necessary curing with dangerous chemicals creates an opportunity for improvements. Furthermore, it is possible to further enhance the physical, chemical and mechanical properties of these resins. Accordingly, there is a need for the production of new resins with enhanced functionality and improved safety which can be used in a wide variety of industrial applications.